1. Field of the Invention
The present invention relates to induction melting. In particular, but not exclusively, the present invention relates to a method and apparatus for melting a target material such as glass in a melting vessel using two or more induction heating coils. The mutual induction of current in a non-energised heating coil adjacent to an energised heating coil is prevented so that the temperature in regions within the melting vessel can be carefully controlled.
2. Related Art
There are many techniques which are known for melting materials and particularly for melting glass like materials. One of these techniques is induction heating in which electrical current is induced to flow in a current conducting melting vessel. These induced currents dissipate energy due to the Joule effect and this heating effect can be used to supply heat to material located within the melting vessel. If enough heat is supplied the material within the melting vessel melts.
A number of uses for such induction heating processes are known. One particular use is in the nuclear industry in which vitrification has long provided a safe long-term conditioning technology for radioactive waste material. In such a situation waste material which may be of low, medium or high-level radioactive waste is mixed together with a glass forming material such as glass frit in the melting vessel. The encapsulation of radioactive waste material within a glassy matrix is chosen because it is a mineral capable of including in its disordered structure many of the elements found in fission-product solutions and other waste material. Once the waste material and glass forming material have melted and been mixed together they may be poured from the melting vessel into a storage canister. The storage canister can be used to define the final solid shape of the glass mixture once it solidifies and can also aid in a subsequent glass conveying process.
An example of this vitrification process is the continuous two-step vitrification process as exemplified by the Marcoule Vitrification Unit (AVM). In this vitrification process two steps are carried out. The first is evaporation-calcination of fission-product solutions. The second step is the vitrification of the resulting calcine. The initial evaporation-calcination step may be carried out with a rotating tube heated to a predetermined temperature. The elements from the input waste material in nitrate or oxide form flow into a second stage induction-heated metal pot. Glass frit or another glass former is added (for example borosilicate glass consisting mainly of SiO2 (silica), B2O3 (boric anhydride), Al2O3 (alumina) and Na2O (sodium oxide) may be used. It is know that fission-product waste material may be incorporated to this glass forming material in quantities ranging from around 10 to 20%. In known vitrification facilities the metal pot is heated to between 1000° C. and 1200° C. using a 200 kW power generator operating at a frequency of 4 kHz. As noted above the glass inside the metal pot is melted by thermal conduction upon contact with the metal wall.
However there are a number of problems which are known with such induction heating systems. One problem is that the induction heating coils which are used to heat the melting vessel are run from high frequency generators which are becoming obsolete. This makes the replacement and/or servicing costs high.
Another problem is that arcing between contactors used to supply power to the heating coils has been observed. The arcing causes failure of the contactors which must be replaced. This is expensive and time consuming.
A further problem is that operating the induction heating elements at a high frequency of around 4 kHz limits control of the temperatures attained in various regions within the melting vessel. This is because penetration depth of a high frequency electromagnetic field limits the depth to which eddy currents are induced and hence limits both the amount of heat generated within the vessel wall and inherently the thickness of vessel that can be used efficiently which in turn limits the life of the vessel due to thermal stresses.